The present invention relates generally to improving accuracy in the detection or determination of initial rotor position in a PMSM (permanent magnet synchronous motor) without use of physical rotor position/speed detection devices or back-EMF sensing techniques, and more particularly to more accurately determining initial rotor position while the rotor is stationary, and still more particularly to more accurately determining initial rotor position without requiring knowledge of variations in specific motor parameters.
It is desirable to know the rotor position in some motor control applications. Accurate determination of rotor position is needed for field orientation control (FOC) of PMSM rotors to achieve accurate, efficient motor operation. Rotor position can be acquired either from physical devices such as an encoder or by use of back-EMF sensing techniques.
A physical sensor such as a Hall effect sensor is commonly used to detect the rotor position, which may be used in a motor controller algorithm. The mounting accuracy of such sensors significantly limits the accuracy of the rotor position determined using such sensors. Use of sensorless motors avoids rotor position inaccuracies caused by sensor mounting inaccuracies. Prior sensorless rotor position sensing techniques are based on electrical voltage and current signals and therefore require the motor to be running in order to determine initial rotor position. If the rotor is at a standstill, there is no simple, practical way of determining the rotor position. For example, there is no back EMF signal which can be used to detect rotor position if the motor is not running. Prior sensorless techniques for determining rotor position (for example as described in U.S. Pat. No. 7,334,854 “Sensorless Start-up Method for Driving a Brushless DC Motor” issued to Chang et al. on Feb. 26, 2008) work only on one kind of motor rather than a variety of kinds of motors and are relatively inaccurate because of motor-to-motor parameter variations.
Hall-effect sensors are employed in many large motor applications (e.g., for powering vehicles) to determine rotor position which may be used in a motor controller algorithm. However, Hall effect sensors are expensive and generally are unreliable so it is desirable to eliminate them from motor controllers. The most unreliable part of a motor control system usually is its rotor position detector/speed control device, and since two or three rotor position detector/speed control devices are frequently needed, a sensorless approach to determining rotor position may substantially reduce cost and substantially improve reliability of remote control systems. Consequently, it would be very desirable to have a sensorless technique for determining rotor position, especially initial rotor position.
Although use of Hall-effect sensors has been successfully eliminated from many small motor applications (e.g., powering hard disk drives), the approach of eliminating Hall-effect sensors is not suitable for large motor applications because of various problems, including problems with maintaining initial rotor position at motor startup. Furthermore, if the rotor is stationary these methods can not be applied either in conjunction with sensor-based motor circuits in conjunction with non-absolute position transducers.
Commonly assigned patent application Ser. No. 13/009,538 filed Ser. No. 13/009,538 entitled “Initial Position Detection for A Sensorless, Brushless DC Motor” by Xiaoyan Wang, incorporated herein by reference, discloses a sensing circuit and a microcontroller having a memory with a lookup table (LUT) stored therein, wherein the microcontroller generates 2N voltage pulses for 2N pairs of phases of a sensorless, brushless direct current (DC) motor having N phases, and wherein the microcontroller is coupled to the sensing circuit so as to determine a phase inductance from a current for each of the 2N pairs of phases of the DC motor, and wherein the microcontroller determines an initial position of the DC motor from the LUT by using the phase inductance from the current for each of the 2N pairs of phases of the DC motor.
A motor controller algorithm may need to utilize parameters of the motor, and this may be problematic if the algorithm needs to provide accurate control of a number of different motors, especially different types of motors, because of significant motor-to-motor differences in corresponding parameters of the various motors. It would be highly desirable for a PMSM motor control system to be more “robust” than the prior art in that the motor control system could economically and accurately calibrate every individual PMSM motor to provide accurate initial rotor position independently of motor-to-motor parameter and temperature differences and also independently of PMSM motor type. It also would be highly desirable for a PMSM motor control system to be more “intelligent” than the prior art in that the motor control system could automatically provide such robust calibration.
The closest prior art is believed to include U.S. Pat. No. 5,191,270 “Method for Starting a Motor” issued to McCormack Mar. 2, 1993; U.S. Pat. No. 7,072,778 “Method and System for Determining a Rotor Position in a Wound Field DC Motor” issued to Swanson Jul. 4, 2006; and U.S. Pat. No. 7,334,854 “Sensorless Start-up Method for Driving a Brushless DC Motor” issued to Chang et al. On Feb. 26, 2008.
Thus, there is an unmet need for a PMSM initial rotor position determination system which operates more accurately than prior initial rotor position detection systems.
There also is an unmet need for a PMSM initial rotor position determination system which operates more efficiently than prior initial rotor position detection systems.
Also is an unmet need for a system and method for accurately determining the initial position of a motor when its rotor is stationary.
There also is an unmet need for a PMSM initial rotor position determination system which is more economical than prior initial rotor position detection systems.
There also is an unmet need for a PMSM initial rotor position determination system which is more robust than prior initial rotor position detection systems.
There also is an unmet need for a PMSM initial rotor position determination system which is more “intelligent” than prior initial rotor position detection systems.